The team analysed feathers from 54 birds kept in museums at Harvard University and the University of Washington in Seattle, US.

The oldest samples are 120 years old.

There was no trend in overall mercury concentrations over time.

But the level of methylmercury - a toxic form of the metal, formed often by bacteria, did show a rise.

Methylmercury is easily absorbed by marine lifeforms such as small fish; and predators of those lifeforms, such as birds, can end up with big concentrations in their tissue.

It can cause developmental defects in humans, and there is evidence that it can damage reproduction in birds and fish.

"People have looked at mercury levels using museum specimens before, but mostly in the Atlantic," said Scott Edwards, a biology professor at Harvard who also curates the university museum's ornithology collection.

"Ours is one of the first to look at patterns in the Pacific basin; this has the largest number of seabird colonies, has the most endangered colonies, and is under severe threat from mercury emissions from Asia."

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About half of the mercury going into the atmosphere comes from natural sources such as volcanoes.

Of the other half, the biggest source is coal-burning, with mercury ocurring as a trace element in many coal deposits.

Although the largest historical sources were North America and Europe, Asia now contributes about two-thirds of the total from energy and industry.

Two years ago, governments agreed to establish a global treaty to restrict mercury emissions, a process due to be finalised in 2013.

Hooked

The pattern of rising methylmercury concentrations in the feathers is, the researchers suggest, a reflection of rising levels in the environment.

But older samples also contained mercury that had apparently been a component of preservative chemicals applied by curators - chemicals that have largely now been phased out.

Museums have not only kept feathers, but also skulls and skeletons, of these big birds

The scientists would now like to work with living albatrosses to answer some outstanding questions on mercury's impact.

"We don't have any direct evidence that levels of mercury were impacting their reproductive success," said Professor Edwards.

"What we'd like to do next is to measure levels live albatrosses, and see whether there's a correlation with the chances of seeing their eggs hatching.

"They're fantastic birds, and a very tractable species to study, so I'd love to do some more work with them."

Like many species of albatross, the black-footed - found only in the northern Pacific Ocean - has been badly affected by longline fishing, where vessels trail lines of hooks tens of kilometres long behind them, the hooks baited to attract high-value predatory fish such as tuna and marlin.

The bait also attract albatrosses and turtles, and longlining is a leading cause of mortality in both, though a number of countries have introduced legislation mandating the use of hooks that are safe for turtles and birds.

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